Use utils.StringWidth to optimize rendering performance

runewidth.StringWidth is an expensive call, even if the input string is pure
ASCII. Improve this by providing a wrapper that short-circuits the call to len
if the input is ASCII.

Benchmark results show that for non-ASCII strings it makes no noticable
difference, but for ASCII strings it provides a more than 200x speedup.

BenchmarkStringWidthAsciiOriginal-10            718135       1637 ns/op
BenchmarkStringWidthAsciiOptimized-10        159197538          7.545 ns/op
BenchmarkStringWidthNonAsciiOriginal-10         486290       2391 ns/op
BenchmarkStringWidthNonAsciiOptimized-10        502286       2383 ns/op

pkg/utils/formatting_test.go

@@ -4,6 +4,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/mattn/go-runewidth"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -250,3 +251,27 @@ func TestRenderDisplayStrings(t *testing.T) {
 		assert.EqualValues(t, test.expectedColumnPositions, columnPositions)
 	}
 }
+
+func BenchmarkStringWidthAsciiOriginal(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		runewidth.StringWidth("some ASCII string")
+	}
+}
+
+func BenchmarkStringWidthAsciiOptimized(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		StringWidth("some ASCII string")
+	}
+}
+
+func BenchmarkStringWidthNonAsciiOriginal(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		runewidth.StringWidth("some non-ASCII string 🍉")
+	}
+}
+
+func BenchmarkStringWidthNonAsciiOptimized(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		StringWidth("some non-ASCII string 🍉")
+	}
+}